1. Field of the Invention
The present invention relates to a capacitor comprising a capacitor electrode with a high-dielectric-constant film and a method for manufacturing the capacitor.
2. Description of the Related Art
DRAMs have recently been increasingly miniaturized. Thus, a high-dielectric-constant film is required for an F40-nm generation (a generation for a 40-nm design rule and subsequent generations) as a dielectric film for a capacitor. For example, SrTiO3 is proposed as major candidates for the high-dielectric-constant film.
On the other hand, TiN/Ti electrodes have conventionally been used as capacitor electrodes. However, when the above-described high-dielectric-constant film is used as a dielectric film, a band offset between a conduction band of a dielectric and a Fermi energy of the electrode decreases. Thus, schottky conduction disadvantageously increases leakage current. Consequently, using the TiN/Ti capacitor electrode with the high-dielectric-constant film has been difficult. Thus, an electrode with a large work function value has been required for the capacitor with the high-dielectric-constant film. In recent years, efforts have therefore been made to examine electrode materials.
For example, Pt is a material with the highest work function, However, Pt offers insufficient processability and is insufficiently practical for production of semiconductor devices such as DRAMs. An electrode material offering high processability needs to be used. Thus, efforts have been made to develop an electrode material other than Pt.
C. M. Chu, et. al., Symp. On. VLSI Tech, Dig., 2001, T4B-3 discloses a capacitor with an MIM structure (Metal-insulator-Metal; a capacitor structure with an upper electrode and a lower electrode each formed of a metal film) of Ru/SrTiO3/Ru. For the capacitor in C. M. Chu, et. al., Symp. On. VLSI Tech, Dig., 2001, T4B-3, the MIM structure of Ru/SrTiO3/Ru is produced and then thermally treated to improve the crystallinity of the capacitor (SrTiO3). This reduces the leakage current value.
U.S. Pat. No. 5,520,992 discloses a capacitor electrode using a high-dielectric-constant film. According to U.S. Pat. No. 5,520,992, a conductive oxide/barrier metal/sub (substrate; semiconductor substrate) is formed as a lower electrode, and a high-dielectric-constant film is formed on the conductive oxide/barrier metal/sub. Then, a barrier metal/conductive oxide is formed on the high-dielectric-constant film in the high-dielectric-constant film/conductive oxide/barrier metal/sub structure as an upper electrode. Therefore, the capacitor in U.S. Pat. No. 5,520,992 has a barrier metal/conductive oxide/high-dielectric-constant film/conductive oxide/barrier metal/sub structure. Here, RuOx or the like is used as the conductive oxide, and nitride such as RuN or the like is used as the barrier metal.
However, I have now discovered that Ru, the electrode material used in C. M. Chu, et. al., Symp. On. VLSI Tech, Dig., 2001, T4B-3, is easily oxidized to vary both resistivity and work function of Ru, which thus becomes unstable (degraded heat resistance). Therefore, a reduction in leakage current value is limited as long as the single Ru film is used as a capacitor electrode. There has thus been a demand for a stable electrode material with a high work function.
Furthermore, I have now discovered that RuOx, used in C. M. Chu, et. al., Symp. On. VLSI Tech, Dig., 2001, T4B-3 as conductive oxide, exhibits a work function equivalent to that of Ru metal (at most 5.1 eV). With such a conductive oxide, an oxygen defect in the film causes conduction. Thus, the resistivity and work function of the film vary according to the concentration of oxygen in the film. Moreover, when a high-dielectric-constant film for a capacitor is formed, thermal treatment is required in order to improve performance of the resulting capacitor. During the thermal treatment, a variation in oxygen concentration may vary the resistivity and work function of RuOx, thus significantly degrading heat resistance.
As described above, the proposed capacitor electrodes do not use a low-electric-resistance electrode material offering a high work function and appropriate heat resistance.
Thus, as a result of keen examinations, the present inventor has found that a film with oxygen atoms and nitrogen atoms distributed therein so as to exhibit a particular concentration distribution can be used as a low-electric-resistance electrode material for the capacitor electrode which offers a high work function and appropriate heat resistance.